Today's highly advanced information society has dramatically developed, and particularly, “mobile communication devices” are being widely spread in the general public. The great demand for “mobile communication devices” is regarded as one of the essential elements in the future semiconductor industry. To satisfy the demand, however, it is necessary to achieve non-volatility of information, as well as high-speed performance, lower electric consumption, and large capacities that have conventionally be required in semiconductor integrated circuits. In response to such demands, attention has been drawn to a novel memory device in which the ferromagnetic storage technique that excels in non-volatile high-density recording combined with the semiconductor integration electronics. This device is called a magnetic random access memory (hereinafter referred to as “MRAM”), and a magnetic tunnel junction (hereinafter referred to as “MTJ”) having a thin insulating tunnel barrier sandwiched between ferromagnetic electrodes is used as a memory device for the MRAM (disclosed in “Present and Future of Magnetic RAM Technology”, K. Inomata, IEICE Trans. Electron. Vol. E84-C, pp. 740-746, 2001, for example).
In the MTJ, the tunneling resistance differs depending on the relative magnetization direction between the ferromagnetic electrodes. This is called a tunneling magneto-resistance (TMR) effect. Utilizing TMR, it is possible to electrically detect the magnetizing state of each ferromagnetic body. Accordingly, the information non-volatile storage technique using ferromagnetic bodies can be ideally incorporated into the semiconductor integration electronics by virtue of the MTJ.
In the following, an example of the conventional technique is described in conjunction with FIG. 10. As shown in FIG. 10, in a MRAM memory cell 100, a 1-bit memory cell is formed with a MTJ 101 and a metal-oxide-semiconductor field-effect transistor (hereinafter referred to as “MOSFET”) 103. The MTJ 101 is a tunnel junction that is formed with a first ferromagnetic electrode 105, a second ferromagnetic electrode 107, and a tunnel barrier (an insulator) 108 formed with an insulator provided between the first and second ferromagnetic electrodes 105 and 107.
The source (S) of the MOSFET 103 is grounded (GND), and the drain (D) of the MOSFET 103 is connected to the ferromagnetic electrode 107 of the MTJ 101 with a plug PL or the like. The ferromagnetic electrode 105 of the MTJ 101 is connected to a bit line BL. A rewrite word line 111 is disposed to cross the bit line BL immediately above or below the MTJ 101, being electrically insulated from the MTJ 101 and the other lines by the insulating film 115. A read word line WL is connected to the gate electrode G of the MOSFET 103.
Since the magnetization direction can be maintained in a non-volatile manner in a ferromagnetic body, binary information can be stored in a non-volatile manner by adjusting the relative magnetization state between the ferromagnetic electrodes of the MTJ to parallel magnetization or antiparallel magnetization. In the MTJ, the tunneling resistance differs depending on the relative magnetization state between the two ferromagnetic electrodes, due to the TMR effect. Accordingly, the magnetization state in the MTJ can be electrically detected, using the tunneling resistance that depends on the magnetization state such as parallel magnetization and antiparallel magnetization.
To rewrite information, the retentivity of the ferromagnetic electrode 105 is made different from the retentivity of the ferromagnetic electrode 107 in the MTJ 101, or the magnetization of the ferromagnetic electrode with lower retentivity or an unfixed magnetization direction is inverted while the magnetization direction of the other ferromagnetic electrode is fixed. Hereinafter, the ferromagnetic electrode having the magnetization varied will be referred to as the “free layer”, and the ferromagnetic electrode having the fixed magnetization will be referred to as the “pin layer”. More specifically, currents are applied to the bit line BL and the rewrite word line 111 that cross each other on the selected cell, and the magnetization state of the MTJ 101 in the memory cell 100 selected by the compound magnetic field of the magnetic fields induced by the currents is changed to parallel magnetization or antiparallel magnetization. Here, the size of each current to be applied to each corresponding line is set so that the magnetization of each MTJ 101 of the unselected cells having the same bit line BL or the rewrite word line 111 as that of the selected cell is not inverted by the magnetic fields generated from only either the bit line BL or the rewrite word line 111. To read information, a voltage is applied to the read word lines WL connected to the selected cell so as to energize the MOSFET 103, and a read driving current is then applied to the MTJ 101 via the bit line BL. In the MTJ 101, the tunneling resistance differs depending on the magnetization state such as parallel magnetization or antiparallel magnetization, due to the TMR effect. Accordingly, the magnetization state can be checked by detecting a voltage drop (hereinafter referred to as “output voltage”) caused by the read driving current in the MTJ 101 (see “Present and Future of Magnetic RAM Technology”, K. Inomata, IEICE Trans. Electron. Vol. E84-C, pp. 740-746, 2001).